In the printing process using printing ink of a liquid form, when printing paper sheets are stacked one over the other immediately after printing, the printing ink deposited on one sheet of printing paper may adhere to the overlying sheet of printing paper, or so-called offsetting may occur. Also, if the printed image is touched by a finger or the like immediately after printing, the printed image may be smeared. If the deposited ink is excessive, the ink may penetrate the printing paper, and may show on the other side of the paper sheet or, in other words, strike-through may occur. These problems are particularly serious in stencil printing where the amount of ink or the ink deposit for forming the printed image on the printing paper is relatively great.
Conventionally, to avoid the occurrence of offsetting and strike-through, various measures have been taken, such as reducing the amount of ink deposit for forming a printed image on the printing paper during the printing process, and speeding up the drying of the printing ink by heating the printing ink deposited on the printing paper.
When printing paper is conveyed to an ejected paper tray or a sorter upon completion of printing, the conveying rollers cannot engage the front face of the printing paper with printing ink deposited thereon because the printed image must be protected from contact. Therefore, conventionally, printing paper was conveyed without touching the front face of the printing paper carrying the printed image by using a conveying mechanism such as a belt conveyor which acts only upon the reverse surface (the face not carrying any printed image) of the printing paper. Such a printing paper conveying device is disclosed, for instance, in Japanese patent laid open (kokai) publication No. 50-88769.
Attempts to prevent offsetting and strike-through in the printing process by reducing the amount of ink deposit give rise to Various problems. Controlling the amount of ink deposit so as to prevent strike-through is highly difficult, and excessive reduction in the amount of ink deposit causes degradation of image quality, such as insufficiency in print density.
Attempts to prevent offsetting and strike-through by drying the printing ink also cause various problems. Because the process of drying the printing ink has to be completed in a short time period while the printing paper is being ejected out of the printing device, and a heater having a relatively large capacity is required. This problem becomes progressively more pronounced as the printing speed increases. From a practical point of view, it is not possible to dry the printing ink in a short time period to a sufficient extent to prevent the occurrence of strike-through.
When printing paper is conveyed without touching the printed surface of the printing paper, the capability to stack the printing paper in a neat fashion on the paper ejection tray and the sorter cannot be attained so easily as in the case of the normal PPC (plain paper copier) or the like in which the copy paper is fed by contacting each sheet from two sides. This also becomes more pronounced as the printing speed or, in other words, the paper ejecting speed increases. Furthermore, the freedom in selecting the path of ejecting printing paper is more restricted.